In the 3rd Generation Partnership Project (3GPP) RAN1 #70 in August 2012, it was agreed to introduce a new transmission mode (TM10) to Rel 11 of Long Term Evolution-Advanced (LTE-A). The purpose of introducing TM10 is to support CoMP operation, which needs to know additional information than single cell operation for UE demodulation process.
Those additional information that should be dynamically signaled (hence has to be in Downlink Control Information (DCI)) includes at least the Cell-specific Reference Signal (CRS) pattern that UE needs to rate match and the assumption of which Channel State Information-Reference Signal (CSI-RS) ports should be assumed to be quasi-colocated with Demodulation Reference Signal (DMRS) ports. There could be some other dynamic information such as Multicast-Broadcast Single Frequency Network (MBSFN) subframe configuration of each transmission point (TP), and zero power CSI-RS of each transmission point. However, this disclosure focuses on CRS pattern and antenna quasi-colocation because those two are the most agreeable dynamic signaling at this stage.
To reduce overhead, CRS pattern and antenna quasi-colocation should not be purely in DCI. Instead, several candidates of CRS pattern and antenna quasi-colocation are configured in Radio Resource Control (RRC). In DCI, UE may select among those candidates. Such method could maintain dynamic behavior while keeping lowest DCI overhead.
There are a couple of methods for higher layer (e.g. RRC or Medium Access Control (MAC)) signaling. One method is to have candidates jointly for CRS pattern and antenna quasi-colocation. For example, candidate A corresponds to {2 CRS ports and frequency shift 1, first CSI-RS is quasi-colocated with DMRS}, while candidate B corresponds to {4 CRS ports and frequency shift 0, second CSI-RS is quasi-colocated with DMRS}, etc. Those may be called as CoMP state (hereinafter also simply referred to as state) A and B, respectively.
Another higher layer signaling is to configure separate candidates for CRS pattern and antenna quasi-colocation. For example, three candidates for CRS patterns are {2 ports and shift 1, 4 ports and shift 0, 2 ports and shift 2}. The candidates for antenna quasi-colocation are {1st CSI-RS colocated, 2nd CSI-RS colocated, 3rd CSI-RS colocated}. UE may use separate DCI bits to select the candidates. However, the overhead may be concerned (at least 3 bits). A possible approach for this higher layer signaling is to define certain combination of CRS pattern and antenna quasi-colocation in specification, and use DCI to select among those combinations.
Both higher layer signaling design mentioned above are feasible. In this disclosure, we use the first signaling design as an example. But it should be understood that the spirit of the present disclosure can be simply extended to the second higher layer signaling design.
Although there are totally up to 12 possible states (4 CRS patterns, and 3 antenna quasi-colocations), 12 candidates in higher layer is not necessary because CRS pattern and antenna quasi-colocation are correlated, i.e., possibly a first CRS pattern would like to imply a first CSI-RS quasi-colocation. Therefore, in general 4 states are sufficient for most CoMP operations. The next question is which codepoints in DCI can be used to indicate those states.
In general it is preferred to keep DCI size small to avoid Physical Downlink Control Channel (PDCCH) capacity overload. This is especially true for CoMP operation because CoMP operation is more likely to be in cell edge (low Signal-to-Noise Ratio (SNR) region), where the DCI may take larger resources than cell center (or high SNR region).
In Rel-10 TM9, DCI 2C was introduced for single cell operation. In Rel-11 TM10, DCI 2D should be at least no larger than DCI 2C because TM10 is to support CoMP operation. Therefore, it is not preferred to add new bits in DCI which increase DCI size. It is also not preferred to use optional field in DCI 2C because it will increase DCI size as well.
Therefore, it is preferred to reuse/revise the existing codepoints in DCI 2C to signal CoMP states. In Rel-10 DCI 2C, the DMRS antenna ports, scrambling identity, and transmission rank (number of layers) are signaled in an 8-value indicator as in the following table:
TABLE 1Rel-10 DCI 2C designOne Codeword:Two Codewords:Codeword 0 enabled,Codeword 0 enabled,Codeword 1 disabledCodeword 1 enabledValueMessageValueMessage01 layer, port 7, nSCID = 002 layers, ports 7-8, nSCID = 011 layer, port 7, nSCID = 112 layers, ports 7-8, nSCID = 121 layer, port 8, nSCID = 023 layers, ports 7-931 layer, port 8, nSCID = 134 layers, ports 7-1042 layers, ports 7-845 layers, ports 7-1153 layers, ports 7-956 layers, ports 7-1264 layers, ports 7-1067 layers, ports 7-137Reserved78 layers, ports 7-14
The two columns in table 1 are determined based on if two transport blocks (TB) are enabled or one TB is enabled, i.e. if two codewords are enabled or one codeword is enabled. If only one TB is enabled, left column is determined. If two TBs are enabled, right side column is determined.
The method to determine if a TB is disabled or enabled is based on the relevant Modulation and Coding Scheme (MCS) and Redundant Version (RV) of the TB. If IMCS=0 and rvidx=1, then the corresponding TB is disabled. Otherwise the TB is enabled. If only one TB is enabled, the TB is mapped to CW1. If two TBs are enabled, TB1 and TB2 are mapped to CW1 and CW2 respectively.
Moreover, for value 4, 5, 6 of the 8-value indicator in the first column, they are used for retransmission of a TB if the TB is transmitted by 2, 3, 4 layers in the initial transmission, respectively.